1. Field of Invention
Aspects of the invention relate to a system for using a sensor to monitor both the shape of an arm as well as an external force applied to that arm. Some aspects of the invention are particularly suited for monitoring flexible guide tubes and articulated arms used in robotic surgery.
2. Art
There has been considerable effort in developing systems for performing minimally invasive surgery. One area of particular interest is robotically assisted surgery.
In robotically assisted surgery, the surgeon typically operates a control device at a location that is remote from the patient to control the motion of surgical instruments at the patient's surgical site. The control device typically includes one or more manually operated input devices, such as multiple degree of freedom master tool manipulators, joysticks, exoskeletal gloves, or the like, which are coupled to the surgical instruments via servo motors for articulating the instruments at the surgical site. During the surgical operation, the control device controls a surgical robotic manipulator that provides mechanical articulation and functional control of a variety of surgical instruments, such as tissue graspers, needle drivers, electrosurgical cautery probes, etc., that each perform various functions for the surgeon, e.g., holding or driving a needle, grasping a blood vessel, dissecting, cauterizing, or coagulating tissue.
Such systems typically include at least one arm having a plurality of joints that interconnect small links to provide articulation. Some form of control mechanism is provided to move the arm into various poses. For example, the control mechanism can include one or more tendons (e.g., cables) running along the length of the arm. Tensioning one or more of the tendons causes the arm to bend at the joints. The tendons may actively control the arm's bending and straightening. Alternatively, in some designs, each joint can be provided with a stiffening element (e.g., a spring) that provides a restoring force to return the arm to a straight orientation when tension on a tendon has been relaxed. Some practical designs may contain multiple bending links per joint. In such multiple-link joints, the stiffening element controls how the multiple bending links bend in a coordinated fashion to form the complete joint.
Although there is a direct relationship between the amount of tension placed on the tendons and the resulting shape of the arm, mechanical tolerances, drive train friction, tendon stretch, and other conditions may prevent one from determining the actual shape of the arm with sufficient precision if the determination is based solely on cable tension. Therefore, various efforts have been made to develop a system to continuously monitor the actual shape of the arm as it moves during a surgical procedure.
One such monitoring approach is disclosed in U.S. Patent Application Pub. No. US 2007/0156019 A1 (filed Jul. 20, 2006) (the “'019 Application”), which is incorporated herein by reference. This approach relies on a fiber optic shape sensor. In this type of device, an optical fiber is provided with a plurality of cores. Arrays of Bragg gratings are formed along the core continuously or at spaced-apart locations. Each Bragg grating comprises a series of modulations of the core's refractive index so as to generate a spatial periodicity in the refraction index. The spacing may be chosen so that the partial reflections from each index change add coherently for a narrow band of wavelengths, and therefore they reflect only this narrow band of wavelengths while passing a much broader band. During fabrication of the gratings, the modulations are spaced by a known distance, thereby causing reflection of a known band of wavelengths. When a strain is induced on the fiber core, the spacing of the modulations will change, depending on the amount of strain in the core.
To measure strain, light is sent down the fiber, and the reflected wavelength is a function of the strain on the fiber and its temperature. This fiber Bragg grating (FBG) technology is commercially available from a variety of sources, such as Smart Fibres Ltd. of Bracknell, England. When applied to a multicore fiber, bending of the optical fiber induces strain on the cores that can be measured by monitoring the wavelength shifts in each core. By having two or more cores disposed off-axis (i.e., not coincident with the lengthwise center longitudinal axis) in the fiber, bending of the fiber induces different strains on each of the cores. These strains are a function of the local degree of bending of the fiber. Regions of the cores containing FBGs, if located at points where the fiber is bent, can thereby be used to determine the amount of bending at those points.
The collected data, combined with the known spacings of the FBG regions, can be used to reconstruct the shape of the fiber. Such a system has been described by Luna Innovations, Inc. of Blacksburg, Va.
The '019 Application describes how this technology can be used to monitor the shape of a robotic arm. As discussed therein, a control system can be provided for detecting the position of the surgical instrument and for utilizing that information to assist in surgical procedures. In one embodiment, the control system includes a detection system and a servo controller. The detection system is utilized for generating and detecting the light used for determining the position of the instrument. The servo controller may utilize the position information as feedback for positioning the instrument.
When using an articulated arm during surgery, it is also desirable to know about any external forces placed on the arm, particularly external forces applied at the distal tip. Information about the external forces upon the arm can be fed back to the surgeon during the procedure to facilitate the manipulation of the arm.
Various approaches have been developed for monitoring the external forces placed upon an articulated arm. In one example, strain gauges are placed on rigid portions of the arm. External forces induce strain in the gauges that can be measured. Examples of the use of force sensors in robotic arms can be found in the '019 Application, U.S. Patent Application Pub. No. US 2008/0065111 A1 (filed Sep. 29, 2007), U.S. patent application Ser. No. 11/858,772, (filed Dec. 18, 2007), and U.S. patent application Ser. No. 12/060,004, (filed Mar. 31, 2008), all incorporated herein by reference.